When is a Mac excluded from a this feature? Is it a hardware limitation?

I ask because I learned that AirDrop for example is excluded from some Macs as they don't support the ad-hoc Wi-Fi networking which is required to use AirDrop over Wi-Fi.

I heard that Power Nap might require Intel‘s Smart Connect Technology and is therefore excluded from most Macs. However, this is unlikely to be true as the MacBook Air (Late 2010 Mid 2011 or newer) is already listed as supported - but Intel's Smart Connect was just about to be announced around 2011.

Some 2010 hardware, for which firmware was apparently seeded by Apple (example: MBA13SMCSeed_2010.smc within publicly available 2010MBA13.pkg), is currently unlisted under Power Nap in the specifications.

Smart Connect is a marketing term, and apparently a Windows marketing term, at that. Clearly Power Nap uses a technology akin to Smart Connect. Whether it's called Smart Connect (it's not) isn't the issue. Nor is it clear that Smart Connect is specifically a processor feature — Intel's marketing indicates it involves interaction between the BIOS and the WiFi card, although I'm sure the processor's ability to operate at low power is also a factor.
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Daniel♦Jul 27 '12 at 15:51

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@DanielLawson Some guys are working to get PowerNap working on the 2010 MBA. This guy reports that it works. This would suggest that PowerNap is not relying on Smart Connect as Smart Connect was introduced in 2011.
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gentmattJul 27 '12 at 15:54

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This sounds very Apple. If a feature works but not well enough or consistently enough to meet their quality standards, Apple tends to not offer the feature. This fits my heat theory rather than an absolute "this chipset does or does not support it" — if PN can be made to run on hardware but it triggers the cancelation of PN too often (for example, because of excess power draw or too much heat), Apple seems inclined to turn it off altogether for that model.
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Daniel♦Jul 27 '12 at 15:58

4 Answers
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Only the MacBook Air (Mid 2011 or newer), MacBook Pro with Retina display were originally supported supported for Power Nap. The reason for this is An SMC update is required to gain all the functionality of Power Nap, and these are the only models Apple has chosen to actively write the firmware updates to support the feature. In other words, it's not that Apple runs some exclusion script to keep other models out; it takes an active update to the firmware to make a model run Power Nap, and they've chosen to do that for two particular lines of computers.

In a subsequent firmware update, Apple included Mid 2010 MacBook Air models in Power Nap.

As far as why they've made that decision, we enter the realm of speculation, but fortunately, we have some good information here.

The idea behind Power Nap is that when the computer appears to be asleep, from time to time, it actually wakes up and performs various tasks. This waking, however, is not noticeable to the user because the computer remains dark and silent. This means no audio or video is activated. It also means, of particular relevance to this question, the fan is also kept off.

This requires a storage device that can run without a fan and a processor that can run without a fan (sorry notoriously hot G5 — I doubt there would ever be a way to support you for this).

According to John Siracusa's epic Mountain Lion review, the Dark Wake mode that Power Nap invokes does not turn on the fans. Thus, only devices Apple knows are designed to be able to access their storage without risk of overheating (as a spinning hard drive might) will take advantage of Power Nap. Siracusa suggests that third party SSDs might have a different heat profile.

While SSDs could be installed in any computer, and Apple could perhaps write firmware updates for all their Macs so that Power Nap checks to see if the computer has a SSD drive and if so, runs Power Nap, they haven't chosen to do so. Instead, they've made Power Nap work only on those models of computer that they know will have a SSD drive.

The limitation might not be drive-based; it could be the processor that can't run silently. This seems slightly less likely to me, given the supported models, but it could explain why the 2010 MacBook Air is excluded. Power Nap monitors temperature, and if the system gets too hot to run without cooling, Power Nap disengages, but if the processor in earlier MBAs was triggering such an alert too often, that may be why Apple removed them from the list of models that support Power Nap.

This still doesn't explain why Apple won't support third-party SSDs. Is it, that Apple will not trust them because the quality varies quite a bit between the different manufacturers?
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gentmattJul 25 '12 at 20:43

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I'd imagine the question belongs the other way around. They do support their own SSDs because they are confident they can run safely without the fan on. They do not (yet?) detect and verify that third party SSDs can safely run silently and dark. First do no harm.
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Daniel♦Jul 25 '12 at 20:45

I believe the linked-to Siracusa review actually mentions that even standard "hard-drive replacement" SSDs may have too-high power requirements; that even Apple-provided SSDs are not supported, only "on-board Flash storage" such as that wired into the newer MacBook Airs.
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Dan JJul 25 '12 at 23:29

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The argument that Apple will not update the firmware because the share of users who will profit may be too low is reasonable. I'm still disappointed as professional products deserve upgrades regardless of such obstacles.
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gentmattJul 27 '12 at 15:13

The why did Apple engineers X question is always a hard one to peg, but the simple answer is that a very small subset of Mac hardware is targeted for the initial Power Nap implementation.

Specifically, the Sandy Bridge and newer Air and the retina MacBook Pro all have non-user replaceable RAM and SSD so Apple has great certainty that there will be no oddball ram timing issues or storage related timeouts. Furthermore, these models all have intel integrated graphics and intel chipsets that were designed for this sort of "operation while sleeping" to perform network operations in a low power state.

Macs have long had the ability to wake up using their internal clocks from the Energy Saver preference. Next, they were programmed to wake periodically to maintain DHCP leases and talk with Airport hardware so that the routers could proxy bonjour sharing requests while the Mac slept. Power Nap takes this ability of the network interfaces to remain alert while in a very low power state one step further. Now the CPU, storage and network stack (but not the fans or display and keyboard lights) power up on an hourly basis to see what tasks can be accomplished before returning to sleep.

If you look at what Power Nap does - it is clear that the functions are designed to use as little power as possible:

App Store updates are only triggered weekly

Only one time machine backup is completed while sleeping. After that, it stops trying to back up while that sleep lasts

Software Updates (security) trigger once daily

The rest of the items like Photo Stream, Mail, iCloud documents and synced data are an hourly refresh while sleep + nap continues

Napping activity stops once the battery level has 30% full charge capacity remaining

Even the name Power lets you know that the primary focus is preserving power but still do the things that you will have to do at some point. Others have speculated that heat is a design issue, and in so far as heat comes from using battery power I would agree that heat is a byproduct of the primary design limitation of Power Nap.

Anyone that has used a MacBook Pro in clamshell mode or had one wake up while shut in a bag is aware that these machines run just fine at temperatures that most people are alarmed and worry about light burns / pain while holding.

Only time (or some public statement, lawsuit deposition or leak) will show us which other models gain Power Nap functionality going forward and whether my reasoning is on the mark about why three models only support Power Nap at launch.

And as a brief aside - the tight integration, sweating the details, making sure a feature is stable and will work well in practice (being conservative about what older models could run some new feature) is par for the course and part of what makes Apple Apple. I would expect some enterprising programmer to make a Cat Napper program to introduce an approximation for older Macs. The beauty of Power Nap is that hardware and software collaborate to only wake the most minimal part of the computer. Only Apple engineers know which logic boards and components truly support this minimal wake/nap idiom.
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bmike♦Jul 27 '12 at 16:23

General agreement. Power Nap aside: in general, I should not expect any additional explanation from Apple about support – for any combination of things – being withdrawn or deferred. When behaviours become good enough for things to work without the need for additional explanation, that's when support will expand, I reckon.
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Graham PerrinJul 27 '12 at 16:31

Good answer with good reasoning! I understand that Apple has decided to not ship software when things aren't stable. After all Apple has a guarantee on their products that it needs to fulfill. Hence, stability comes first.
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gentmattJul 27 '12 at 16:38

The Technical Specifications don't just refer to SSDs in general, but specifically mention Macs which have custom designed flash storage. i.e. A Mac with a 2.5" SSD from Apple will not support Power Nap.
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gentmattJul 11 '12 at 19:33

A best answer about Apple's software is always when you get it from them. However, Daniel has found very reasonable argument in the last paragraph: the share of users who will profit may be too low
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gentmattJul 27 '12 at 15:22

My Air from 2011 doesn't have a retina display but happily naps. I did get an EFI firmware update to add support for the rest of Power Nap that wasn't on the machine when it shipped.
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bmike♦Jul 27 '12 at 16:09